A principal method for producing nitrogen from air is pressure swing adsorption (PSA). Carbon molecular sieves and zeolites are the most common adsorbents in PSA systems for production of nitrogen, but the main drawbacks of currently available adsorbents are their high cost of production and final product variability. A major challenge in this area of technology is to improve the structure of carbon molecular sieves in order to attain higher air recoveries, higher nitrogen productivity, and better consistency in adsorbent properties--all at a lower cost. In order to accomplish these goals, improvement in adsorbent properties, such as selectivity and equilibrium adsorption capacity, are needed.
Carbon molecular sieves are effective for separating oxygen from nitrogen because the rate of adsorption of oxygen is higher than that of nitrogen. The difference in rates of adsorption is due to the difference in size of the oxygen and nitrogen molecules. Since the difference in size is quite small, approximately 0.2 .ANG., the pore structure of the carbon molecular sieve must be tightly controlled in order to effectively separate the two molecules. In order to improve the performance of carbon molecular sieves, various techniques have been used to modify pore size. The most common method is the deposit of carbon on carbon molecular sieves. For example, U.S. Pat. No. 3,979,330 discloses the preparation of carbon containing molecular sieves in which coke containing up to 5% volatile components is treated at 600.degree. C.-900.degree. C. in order to split off carbon from a hydrocarbon. The split-off carbon is deposited in the carbon framework of the coke to narrow the existing pores.
Sutt, in U.S. Pat. Nos. 4,528,281, 4,540,678, 4,627,857, and 4,629,476, discloses various preparations of carbon molecular sieves for use in separation of gases. U.S. Pat. No. 4,528,281 discloses impregnating a molecular sieve with a polymer having a molecular weight of at least 400. U.S. Pat. Nos. 4,540,678 and 4,627,857 disclose preparation of carbon molecular sieves by heating a charred naturally occurring substrate or an agglomerated naturally occurring substrate to temperatures of about 500.degree. C. to about 1100.degree. C.
U.S. Pat. No. 4,742,040 discloses a process for making a carbon molecular sieve having increased adsorption capacity and selectivity by pelletizing powder coconut shell charcoal containing small amounts of coal tar as a binder, carbonizing, washing in mineral acid solution to remove soluble ingredients, adding specified amounts of creosote or other aromatic compounds, heating at 950.degree. C.-1000.degree. C., and then cooling in an inert gas.
U.S. Pat. No. 4,880,765 discloses a process for producing carbon molecular sieves with uniform quality and good separating properties by treating a carbonaceous product with inert gas and steam in a vibrating oven and further treating it with benzene at high temperatures to thereby narrow existing pores.
U.S. Pat. No. 5,081,097 discloses copper modified carbon molecular sieves for selective removal of oxygen. The sieve is prepared by pyrolysis of a mixture of a copper-containing material and a polyfunctional alcohol to form a sorbent precursor. The sorbent precursor is then heated and reduced to produce a copper modified carbon molecular sieve.